Friday, 15 June, 2018 UTC

Summary

The role of web animation has evolved from mere decorative fluff to serving concrete purposes in the context of user experience --- such as providing visual feedback as users interact with your app, directing users' attention to fulfill your app's goals, offering visual cues that help users make sense of your app's interface, and so on.

To ensure web animation is up to such crucial tasks, it's important that motion takes place at the right time in a fluid and smooth fashion, so that users perceive it as aiding them, rather than as getting in the way of whatever action they're trying to pursue on your app.

One dreaded effect of ill-conceived animation is jank, which is explained on jankfree.org like this:

Modern browsers try to refresh the content on screen in sync with a device's refresh rate. For most devices today, the screen will refresh 60 times a second, or 60Hz. If there is some motion on screen (such as scrolling, transitions, or animations) a browser should create 60 frames per second to match the refresh rate. Jank is any stuttering, juddering or just plain halting that users see when a site or app isn't keeping up with the refresh rate.

If animations are janky, users will eventually interact less and less with your app, thereby negatively impacting on its success. Obviously, nobody wants that.

In this article, I've gathered a few performance tips to help you solve issues with JavaScript animations and make it easier to meet the 60fps (frame per second) target for achieving smooth motion on the web.

#1 Avoid Animating Expensive CSS Properties

Whether you plan on animating CSS properties using CSS Transitions/CSS keyframes or JavaScript, it's important to know which properties bring about a change in the geometry of the page (layout) --- meaning that the position of other elements on the page will have to be recalculated, or that painting operations will be involved. Both layout and painting tasks are very expensive for browsers to process, especially if you have several elements on your page. As a consequence, you'll see animation performance improve significantly if you avoid animating CSS properties that trigger layout or paint operations and stick to properties like transforms and opacity, because modern browsers do an excellent job of optimizing them.

On CSS Triggers you'll find an up-to-date list of CSS properties with information about the work they trigger in each modern browser, both on the first change and on subsequent changes.

Changing CSS properties that only trigger composite operations is both an easy and effective step you can take to optimize your web animations for performance.

#2 Promote Elements You Want to Animate to Their Own Layer (with Caution)

If the element you want to animate is on its own compositor layer, some modern browsers leverage hardware acceleration by offloading the work to the GPU. If used judiciously, this move can have a positive effect on the performance of your animations.

To have the element on its own layer, you need to promote it. One way you can do so is by using the CSS will-change property. This property allows developers to warn the browser about some changes they want to make on an element, so that the browser can make the required optimizations ahead of time.

However, it's not advised that you promote too many elements on their own layer or that you do so with exaggeration. In fact, every layer the browser creates requires memory and management, which can be expensive.

You can learn the details of how to use will-change, its benefits and downsides, in An Introduction to the CSS will-change Property by Nick Salloum.

#3 Replace setTimeOut/setInterval with requestAnimationFrame

JavaScript animations have commonly been coded using either setInterval() or setTimeout().

Although this works, the risk of jank is high, because the callback function runs at some point in the frame, perhaps at the very end, which can result in one or more frames being missed. Today, you can use a native JavaScript method which is tailored for smooth web animation (DOM animation, canvas, etc.), called requestAnimationFrame().

requestAnimationFrame() executes your animation code at the most appropriate time for the browser, usually at the beginning of the frame.